Curved Planar Reformation (CPR), producing a Curved Planar Reformat, is a technique to provide a representation of an anatomical structure which is curved through a 3D volume. Using conventional display techniques, where a representation of a plane is generated from image slices, problems arise because very few anatomical structures lie in such a plane. For example, the colon has a highly curved shape, so that many planes need to be represented to visualize its structure. Additionally in virtual endoscopy, the user would like to move along an anatomical structure—with conventional display techniques, this would require a continuous repositioning of images as the user moves through the stack of image slices. An example of virtual endoscopy using CPR is described in “Volumetric Curved Planar Reformation for Virtual Endoscopy”, Williams, Grimm, Coto, Roudsari, Hatzakis, IEEE Transactions on Visualization & Computer Graphics, Vol. 14, No. 1, January/February 2008.
In general, CPR visualization may be used to visualize any elongated object of interest in a 3D volume. Typically, CPR is used in CT/MR angiography to visualize arterial and venous vessels, and also in the orthopedic domain to visualize the spinal cord.
FIGS. 3A, 3B and 3C illustrate an example of three steps in a conventional CPR procedure. FIG. 3A depicts a 3D image data volume 200 comprising an elongated anatomical structure. A line 210 is defined along the longitudinal axis of the anatomical structure. A construction plane 220 is defined. FIG. 3B depicts the same 3D image data volume 200 in which a reformat plane 230 has been defined orthogonally to the construction plane 220. The reformat plane 230 also comprises the line 210. FIG. 3C depicts a schematic representation of the curved plane view 240 as provided to the user, in which the line 210 is unfolded onto the screen and the reformat plane 230 is flattened for viewing.
The CPR procedure proceeds as follows. In FIGS. 3A, 3B image data volume 200 is acquired. A line 210 through the anatomical structure is determined—this may be done automatically, or in combination with input from the user. If it is done automatically, it may be advantageous to first segment the contours of the anatomical structure. The determination of the line may include not only the line 210 itself, but also the start and end points of interest. Typically, this line approximates a centerline through the anatomical object. A suitable construction plane 220 is chosen proximate the line 210, either automatically or in combination with input from the user.
As shown in FIG. 3B, a curved reformat plane 230 is then determined, orthogonal to the construction plane 220 through the line 210. Finally, in FIG. 3C, the reformat plane 230 is folded out, and flattened. A representation of the anatomical structure along the line 210 is then displayed to the user on the display. This is a 2D image giving the curved plane view 240. As this line 210 is defined along the anatomical structure, the procedure allows a larger portion of the anatomical structure to be viewed at once.
Other CPR techniques, such as projection CPR, stretched CPR and straightened CPR, are known in the art, such as in the Williams et al. article. What they all have in common is that they “unfold” an elongated anatomical structure onto the screen by determining a curved plane through the structure, and displaying this curved plane.
This reformation process can result in image artifacts, leaving it up to the user to distinguish between artifacts and actual anatomical features.